1- You would need to login to your site
2- Click the administration link to open admin page.
3- hover over 'system admin' and click 'blocks'
4- At bottom of page enter details in ' Add a new block'

5-Block type: select the position on the page.
6- select weight (where it is to appear: 0= high in the page)
7- Visible select 'YES' radio button
8- Visible in: All other modules installed are listed here. Select as req.
9- Title: choose what will be the header of the block.
10- Content: Use a variety of options here, inc. images, links, text, code.
11-Content type. Select according to the tytpe of content inputted.
12-Cache lifetime. Is it static? usually... yes, so set a long time.
13-Preview: click this to see it! edit as required till your happy.
14-Submit: save your changes.

NOW ....

Go to the system admin icon again. This time select 'groups'

1- webmasters and registered groups are usually given automatic access to all content. SO if you want to make your new custom block available ONLY to unregistered anonymous users...

2- Click webmasters group
3- Go to the 'Block access' section for the position you set L, C, R.
4- Find your block title and UNcheck the box relating to it.
5- Update the group (button link at bottom)

6- Now repeat this for the registered group if required
7- Go to anonymous group and CHECK the block to make it visible if required.

NOTE In 'Visible in' ie which modules will display this block you can select all pages (modules) or specific modules in the list. You can also select a combination by holding down the Ctrl key and clicking the selected modules. Make sure you deselect All pages if you do this!!

Please also Xoops operations guide and this FAQ